1. Field of the Invention
The present invention relates to a compact wavelength filter integrated to a single-mode optical fiber, as well as to a method of manufacturing the wavelength filter.
2. Brief Description of the Prior Art
Optical fibers are well-known in the art and are extensively used in telecommunication and control systems, and in sensing and medical apparatuses. The advantages of the optical fibers over the conventional copper conductors and coaxial cables, when used in telecommunication systems, are so significant that eventually the optical fibers will replace the conventional conductors and cables in many applications for the transmission of information signals.
Optical fibers are waveguides capable of propagating visible and/or infrared light. In order to reduce dispersion of the light signals, single-mode fibers are widely used and constitute the most promising type of optical fibers for telecommunication purposes. An advantage of the single-mode optical fibers is their capacity to carry light signals containing several wavelengths simultaneously. However, many applications require that only certain specific wavelengths be transmitted by the fiber; an optical filter is then necessary to eliminate the undesired wavelengths. Applications such as demultiplexing of wavelengths division multiplexed (WDM) light signals, that is the separation of light signals of different wavelengths transmitted by the same fiber, may require a wavelength filter to isolate correctly the different signals to be separated.
In standard WDM telecommunication systems, two wavelengths are used instead of one to double the data transmission capacity. However, special components such as WDM fused couplers are required to launch signals of different wavelengths, usually 1300 and 1550 nm, in a single fiber and to separate them at the receiving end. Ideal WDM couplers would have a coupling ratio of 100% at 1300 nm and of 0% at 1550 nm, or vice-versa. However, such ideal coupling cannot be obtained in practice and the typical isolation achieved is of approximately 17 dB, which does not satisfy the telecommunication requirements (isolation of the order of 35 dB is required). Additional filtering devices are thus necessary to better separate the demultiplexed signals. These devices usually consist of bulky optics components.
Optical filters exist but they suffer from many disadvantages such as poor performance, high complexity, high manufacturing cost, bulkiness, high loss, etc, and in most of the cases they are not an integrated part of the optical transmission media (fiber).
As an example, Canadian patent application number 517,920 filed on Sept. 10, 1986, in the name of Suzanne Lacroix and Frangois Gonthier, proposes a wavelength filter integrated to a single-mode optical fiber. Such a filter comprises several tapers formed onto the fiber which comprises conventionally an opaque outer jacket. To form each taper, a given length of the outer jacket is removed from the fiber, the latter fiber is then heated locally to the point at which it becomes viscous, and the so heated fiber is stretched along its axis. To heat locally the fiber, a very small heat source such as a small flame is used.
Each taper modifies the light signal transmitted through the single-mode fiber, and the plurality of tapers formed in series onto the optical fiber will perform the function of a wavelength filter. By changing the profile of the tapers, as well as the number of such tapers on the fiber, control of the characteristics of the wavelength filter is enabled.
The integrated wavelength filter of Canadian patent application number 517,920 presents the drawback of not being compact. Also, in the infrared region of the light spectrum, the technique involved cannot be applied as its efficiency reduces.
Compact wavelength filters capable of being integrated to an optical fiber and eventually in WDM couplers packages are therefore sought to solve this problem.
In the present disclosure and in the appended claims, the term "light" is intended to encompass both visible and invisible light including of course infrared light, and which may propagate in the wavelength filter in accordance with the present invention integrated to an optical fiber.